Restoring the plant productivity of heavy metal-contaminated soil using phosphate sludge, marble waste, and beneficial microorganisms.

Assisted natural remediation (ANR) has been highlighted as a promising, less expensive, and environmentally friendly solution to remediate soil contaminated with heavy metals. We tested the effects of three amendments (10% compost, C; 5 or 15% phosphate sludge, PS5 and PS15; and 5 or 15% marble waste, MW5 and MW15) in combination with microorganism inoculation (rhizobacteria consortium alone, mycorrhizae alone, and the two in-combination) on alfalfa in contaminated soil. Plant concentrations of Zn, Cu, and Pb were measured, along with proline and malondialdehyde production. The microbiological and physicochemical properties of the mining soil were evaluated. Application of the amendments allowed germination and promoted growth. Inoculation with the rhizobacteria consortium and/or mycorrhizae stimulated plant growth. PS and MW stimulated the production of proline. Inoculation of alfalfa with the rhizobacteria-mycorrhizae mixture and the application of MW allowed the safe cultivation of the legume, as shown by the low concentrations of metals in plant shoots. Zn and Pb concentrations were below the limits recommended for animal grazing and accumulated essentially in roots. Soil analyses showed the positive effect of the amendments on the soil physicochemical properties. All treatments increased soil pH (around 7), total organic carbon, and assimilable phosphorus content. Notably, an important decrease in soluble heavy metals concentrations was observed. Overall, our findings revealed that the applied treatments reduced the risk of metal-polluted soils limiting plant growth. The ANR has great potential for success in the restoration of polymetallic and acidic mining soils using the interaction between alfalfa, microorganisms, and organo-mineral amendments.

Safe Cultivation of Medicago sativa in Metal-Polluted Soils from Semi-Arid Regions Assisted by Heat- and Metallo-Resistant PGPR.

Soil contamination with heavy metals is a constraint for plant establishment and development for which phytoremediation may be a solution, since rhizobacteria may alleviate plant stress under these conditions. A greenhouse experiment was conducted to elucidate the effect of toxic metals on growth, the activities of ROS (reactive oxygen species)-scavenging enzymes, and gene expression of Medicago sativa grown under different metal and/or inoculation treatments. The results showed that, besides reducing biomass, heavy metals negatively affected physiological parameters such as chlorophyll fluorescence and gas exchange, while increasing ROS-scavenging enzyme activities. Inoculation of M. sativa with a bacterial consortium of heat- and metallo-resistant bacteria alleviated metal stress, as deduced from the improvement of growth, lower levels of antioxidant enzymes, and increased physiological parameters. The bacteria were able to effectively colonize and form biofilms onto the roots of plants cultivated in the presence of metals, as observed by scanning electron microscopy. Results also evidenced the important role of glutathione reductase (GR), phytochelatin synthase (PCS), and metal transporter NRAMP1 genes as pathways for metal stress management, whereas the gene coding for cytochrome P450 (CP450) seemed to be regulated by the presence of the bacteria. These outcomes showed that the interaction of metal-resistant rhizobacteria/legumes can be used as an instrument to remediate metal-contaminated soils, while cultivation of inoculated legumes on these soils is still safe for animal grazing, since inoculation with bacteria diminished the concentrations of heavy metals accumulated in the aboveground parts of the plants to below toxic levels.

Characterization of plant growth promoting rhizobacteria and their benefits on growth and phosphate nutrition of faba bean and wheat.

In recent years, more attention has been paid to plant growth promoting (PGP) rhizobacteria use as a biofertilizer alternative to chemical fertilizers, which might cause damage to the environment. The main objective of this work was to evaluate the field application of PGP bacteria and rhizobial strains on the productivity of two food crops extensively used in Morocco; Vicia faba L. and Triticum durum L. A field experiment with four treatments was designed: (1) control without inoculation, (2) PGP bacteria alone (P), (3) rhizobia alone (R) and (4) a mixture of PGP-rhizobia (PR). Furthermore, the PGP strains were tested for their ability to solubilize complex mineral phosphorus and potassium and for their production of indole acetic acid and exopolysaccharides. The strains showed several plant growth promoting traits. Field inoculation by these rhizobacteria improved phosphorus uptake and the agronomic parameters of faba bean and wheat plants, such as biomass of shoots and roots, as well as the weight of bean pods and wheat spikes. The most pronounced effect was displayed by rhizobial strains or the combination of PGP-rhizobia. The rhizobacterial inoculation significantly stimulated the growth of both crops and could be used as potential biofertilizers to optimize growth and phosphorus retention capacity.

Use of Rhizobacteria and Mycorrhizae Consortium in the Open Field as a Strategy for Improving Crop Nutrition, Productivity and Soil Fertility.

Plant growth promoting rhizobacteria (PGPR) and arbuscular mycorrhizal fungi (AMF) are known for their beneficial effects. In recent years, more attention has been paid to their use as biofertilizers to reduce the use of chemical fertilizers causing significant damage to the environment. To have high plant yields, biofertilizers may not be able to sustain plant demands and could be used in combination with chemical fertilizers. However, the application of biofertilizers in the field such as rhizobacteria and AMF are understudied and powerfully needed. In this context, this study aims to evaluate the effect of inoculation with rhizobacteria and AMF and their potential to stimulate two of the most economically important crops in Mediterranean semi-arid areas (Vicia faba L. and Triticum durum L.). The effect of inoculation was studied in field experiment with six treatments: (i) the control without inoculation (C), (ii) PGPR alone (PG), (iii) rhizobia alone (R), (iv) the mixture of PGPR and rhizobia (PR), (v) AMF alone (M), and (vi) the mixture of PGPR, rhizobia and AMF (PRM). The inoculation with the consortium of PGPR-rhizobia-AMF (PRM) induced the greatest effect. This inoculation improved the growth parameters (dry weight of shoots and roots) of faba bean and wheat. An improvement of 130, 200, and 78% was observed in V. faba shoot and root dry weight, and the number of leaves, respectively. Similarly, shoot and root dry weight and number of leaves of T. durum were enhanced by 293, 258, and 87%, respectively. The inoculation improved the productivity of studied plants presented by the number and weight of bean pods (270 × 104 ha-1 and 30737.5 kg.ha-1) and wheat spikes (440 × 104 ha-1 and 10560 kg.ha-1). In addition, the mineral analyses showed that the inoculation with PGPR-rhizobia-mycorrhizae improved N, P, Ca, K, and Na shoots contents, as well as the contents of sugar and proteins. Finally, we revealed the positive impact of the tested biofertilizers and the interest of adoption of innovative practices improving crops productivity and soil fertility.